I suspect it's a combination of their sex work and race. Asian women are often seen as disposable sex objects, even more-so than white women. This is what led him to frequent these places, and then that snowballed into violence against them. However, I do think the fact that they were sex workers was the primary cause of the shooting, and we need to address all aspects of it. First, let's fix the issue with sex work. No more criminalizing any aspect of it. Zero, zip. It's going to happen, let's make it safe. Next, there has been an increase in anti-Asian sentiment since the start of the pandemic, and some of it fuelled by assholes like Trump saying "The China Flu" and "Kung Flu" and so on. We don't need any more racism.
What disgusted me the most was one of the responding police officers trivialized the event - ugh. Stop. Violence against sex workers is super serious.